Lead Variation, Wobble

Gear tooth lead variation caused by wobble.

In this issue we will consider the problem shown in the following gear tooth lead inspection chart (Figure 1).

Figure 1 – Lead inspection with lead variation error

This chart is a measurement along both the left and right flanks of four teeth of a gear. This is an AGMA-standard gear check per the ANSI/AGMA 2015-1-A01 standard. In particular, this is a measurement of the “total helix deviation,” denoted as Fβ, and defined as the “distance between two design helix lines which enclose the actual helix trace over the evaluation range.” In simple words, Fβ is the difference in distance between the measured high and low points. This value is absolute, so viewing the chart is necessary to determine the location of these high and low points. A perfect gear would give a perfectly vertical measurement line (these charts may also be depicted horizontally). By combining the chart and the absolute value of variation, we have both an understanding of the type and magnitude of error to be considered.

Figure 2 – Total helix deviation

Presuming the four teeth are 90° to each other, this chart depicts a lead variation commonly referred to as “wobble.” In this example, the lead of four teeth varies regularly from a plus to a minus condition, indicating wobble. There are four possible sources of this error: